This invention relates to methods of manufacturing a semiconductor device, for example an insulated-gate field-effect power transistor (commonly termed a "MOSFET") of the trench-gate type or an insulated-gate bipolar transistor (commonly termed an "IGBT") of the trench-gate type. The invention also relates to semiconductor devices manufactured by such a method.
Such semiconductor devices are known having source and drain regions of a first conductivity type separated by a channel-accommodating region to which a gate is capacitively coupled, and a localised region of an opposite, second conductivity type which is adjacent to the source region, is contacted by the source electrode and is more highly doped than the channel-accommodating region. A trench-gate device having these features is known from U.S. Pat. No. 5,665,619. The method of manufacture disclosed in U.S. Pat. No. 5,665,619 includes the steps of:
(a) forming at a surface of a semiconductor body a first mask having a first window at a first area of the body where the trench-gate and channel are to be formed, PA1 (b) etching the trench into the body at the first window and providing a gate in the trench where a body region provides the channel-accommodating region, PA1 (c) forming over the gate in the trench a second mask of complementary window pattern to the first mask by providing a differently-etchable material from the first mask in the first window and then etch-removing the first mask from the body while leaving the second mask at the first area where the trench-gate is present, a second area of the body region being present at the complementary window in the second mask, PA1 (d) forming the source region by introducing dopant of the first conductivity type into a part of the second area while masking the trench-gate with the second mask, PA1 (e) forming a localised region of an opposite, second conductivity type by introducing dopant of the second conductivity type into the second area via the second window while masking the trench-gate with the second mask, the localised region being formed to a depth in the body shallower than that of the channel-accommodating region, and PA1 (f) providing a source electrode on the body while masking the trench-gate with the second mask, so as to contact the source region of the first conductivity type and the localised region of the second conductivity type at the surface.
This first mask in U.S. Pat. No. 5,665,619 comprises silicon nitride. The silicon nitride masks underlying areas of the body against oxidation while oxidising an upper part of the gate material to form the second mask of silicon dioxide. This second mask in U.S. Pat. No. 5,665,619 forms a protruding step to the adjacent surface of the body. This step configuration is used in a self-aligned manner to form a further mask with a smaller window, by providing sidewall extensions on the second mask at the step. Thus, U.S. Pat. No. 5,665,619 describes a modified extension of a previously-known trench-gate self-alignment technique, for example as disclosed in U.S. Pat. No. 5,378,655 (our reference PHB 33836). The whole contents of both U.S. Pat. No. 5,378,655 and U.S. Pat. No. 5,665,619 are hereby incorporated herein as reference material. By using such self-aligned techniques as disclosed in U.S. Pat. No. 5,378,655 and U.S. Pat. No. 5,665,619, the number of photolithographic masking steps which require separate alignment can be reduced and compact cellular device structures can be formed.
The localised region of the second conductivity type which is contacted by the source electrode is formed in U.S. Pat. No. 5,665,619 by dopant introduction via the second window, i.e. at a late stage in the manufacturing process. Its localised lateral dimensions are defined by overdoping with a higher dopant concentration of the first conductivity type which is introduced into a part only of the second area in step (d) to form the source region. Thus, the localised region is formed to a shallower depth in the body than both the source region and the channel-accommodating region. However, in terms of improving the blocking/breakdown characteristics of the device, it is advantageous for the localised region to be formed to a greater depth in the body than the channel-accommodating region.